1. Field of the Invention
The present invention relates to methods of generating gas in and foaming well cement compositions during pumping of the compositions.
2. Description of the Prior Art
Foamed hydraulic cement compositions have heretofore been utilized in oil and gas well cementing applications. Typically, a hydraulic cement composition is foamed by combining a mixture of foaming and foam stabilizing surfactants with the cement composition on the surface followed by injecting gas, typically nitrogen, into the cement composition containing the surfactants as the cement composition is pumped into the well bore. This process allows the cement composition to have a gas concentration of from 1% to 99% by volume of the cement composition depending on the downhole pressure and temperature and the amount of gas injected into the cement composition at the surface. However, the equipment required for storing the nitrogen in liquid or gaseous form and injecting it into a cement composition is very elaborate and expensive. In addition, the equipment is frequently unavailable or can not be easily transported to well sites due to their remote locations.
In-situ gas forming agents have been utilized heretofore in well cement compositions to prevent annular gas migration. For example, surfactant coated finely ground aluminum has been included in cement compositions to generate hydrogen gas in the compositions as they are being pumped down well bores and after they are placed in annuluses between the walls of the well bores and casing or other pipe strings therein. The presence of the gas in the cement compositions prevents formation fluids from entering the cement compositions as the cement compositions develop gel strength. That is, the development of gel strength reduces the ability of a cement composition column to transmit hydrostatic pressure. If the hydrostatic pressure of the cement composition column falls below the formation pore pressure before the cement composition has gained sufficient strength to prevent the entry of formation fluids into the well bore, the fluids enter the well bore and form channels in the cement composition column which remain after the cement composition column sets. The presence of the gas which is generated in the cement composition from the finely ground aluminum increases the volume of the cement composition such that the volume increase generated by the gas equals or slightly exceeds the cement composition volume reduction during the development of gel strength due to fluid loss and/or the cement hydration reaction. The increase in volume in the compressibility produced in the cement composition by the gas allows the cement composition column to resist the entry of formation fluids into the well bore.
Other gas forming agents have also been added to well cement compositions to gasify the compositions. For example, U.S. Pat. No. 4,450,010 issued on May 22, 1984 to Burkhalter et al. discloses a well cementing method and gasified cements useful in carrying out the method. That is, U.S. Pat. No. 4,450,010 discloses a method of cementing in subterranean formations using a gasified cement composition which prevents formation fluids from entering the cement composition column formed in the annulus between the well bore and a pipe string therein. The cement composition includes a nitrogen gas generating material, an oxidizing agent and a reaction rate control material whereby a quantity of gas is generated in the cement composition to offset the shrinkage in the cement composition column as it develops gel strength and to provide compressibility thereto whereby the entry of formation fluids into the well bore is reduced or prevented.
While the methods and cement compositions of the prior art have achieved varying degrees of commercial success, a problem which ofen occurs in the use of the methods and cement compositions is that gas is produced in the cement composition prior to and during the pumping of the cement compositions into the well bore which causes gas-locking of the cement pumps. When such gas-locking occurs, the pumps must be shut down and the gas removed during which time additional gas is formed in the cement compositions which causes additional gas-locking when the pumps are again started.